Ok, I found the DLL and StorageDevice Class ... I needed to add a reference to SecretLabls.NetMF.IO to my project .. that's all.
After reading more and more posts about ND+2 and sd card support ... I think I have it clear now that and external SD Card is not supported on the ND+2, since the NetMF only supports 1 SD card, and the ND+2 has one onboard already.
The one that is supported, does not seem to need the Storage Device class, since the mounting of SD card is already handled behind the scenes by the SecretLabs framework ...
Am I right??